The imporatnat steps involved in nitrogen cycle :

(i) Nitrogen fixation - and assimilation of free atmospheric nitrogen by Rhizobium or blue-green algae. 

(ii) Nitrogen is converted to its oxides during lightning and in the prsence of high temperature and pressure. These oxides dissolve to give nitric and nitrous acid and fall on land along with rain. 

(ii) Ammonification -  conversion of ammonia to nitrites by Nitrosomonas (nitrifying bacteria).

(iii) Nitrification - conversion of nitrites to nitrates by Nitrobactor.

(iv) Denitrification - break down of nitrates to free nitrogen by Pseudomonas (denitrifying bacteria).

Growing of  crops year after year results in the depletion of certain nutrients. The soil becomes deficient every year. If these nutrients are not replenished, the soil will become unfit for cultivation and land may become barren. Hence, replenishment of soil is essential.

The two natural ways of soil replenishment are:

(i) Crop rotation i.e., rotation of crops. In this practice, different crops having varied requirements of nutrients are grown on the same land during different seasons. So that the soil is not depleted of any particular nutrient all the time, often a legume crop e.g., pea, gram, pulses etc. are grown alternately to replenish the soil.

(ii) Leaving the agricultural land uncultivated for one or two seasons.This helps the soil to regain its richness.

Following are the measures that one should take for maintaining the fertility of the soil :

(i) Crop rotation - which ensures efficient use of nutrients by all the plants. 

(ii) Growing of leguminous plants as one of the crop which increases the nutrient content of the soil by fixing nitrogen. 

(iii) Leaving the soil uncultivated for one or two seasons.-  This helps the soil to regain its richness.

(iv) Using manure - Being organic in nature it impreoves the fertility and quality of the soil. 

It is very necessary to replenish forests as forest control the following things: 

(i) Rainfall. Trees give out enormous amount of water during transpiration. This water vapour helps in the formation of rain clouds. If trees are killed and not replenished , it will reduce the rainfall in the area 

(ii) Soil erosion. If trees are cut at a large rate the soil becomes uncovered. The top soil which is rich in organic matter will be washed away by water or carried away by wind. It may lead to the formation of a desert.erosion may lead to the formation of desert.

(iii) Carbon dioxide-oxygen balance. Forests have very large number of trees. They give out O₂ and take in CO₂ in day time during photosynthesis. Thus they help in maintaining carbon dioxide-oxygen balance in the atmosphere.

(iv) Natural growth of trees. Forests cannot be grown in a few days or months. Trees take years together to grow their heights.

It is, therefore, necessary to replenish forests at proper time, i.e., we must plan and grow trees and avoid indiscriminate cutting down of forests. 

The points one should know regarding the cycling of material are: 

(i) The source of the element.

(ii) The form in which it is available.

(iii) The form in which it is converted during cycling.

(iv) The form in which it is released into the nutrient pool.